High-Temperature Static Strain
Static strain (DC or mean strain) is usually well understood when assessing gas turbine engine component structural integrity, where calculated load induced strain is combined with laboratory low temperature engine bench static strain measurements and testing information. The largest unknown static strain occurs with assembly and interaction with adjacent engine components, followed by operation induced changes to the baseline FEA analyses. While the inability to measure high temperature static strain is a shortcoming, it is one of the easier parameters to accurately calculate in the engine environment, since nearly all of the static strain encountered in the engine environment is the result of centrifugal loading and/or coefficient of thermal expansion mismatches.
Stress and strain are related by Young’s modulus for the component being instrumented.
In its simplest form: Stress = Strain x Young’s modulus
The basis for needing strain measurements and engine component structural integrity is found in the Goodman-Soderberg Diagram which documents maximum static (mean) strain which is combined with maximum dynamic (alternating) strain. Total strain combines both static and dynamic loading (fatigue), which often leads to a reduction in component life and durability.
Goodman and Soderberg Failure Criteria
The current accepted sensor used for measuring steady state static (mean) strain is the resistance strain gage, which changes electrical resistance with applied strain. The strain gage has been the basis for measuring component life and performance since it was invented in 1938 by Dr. Arthur Ruge and Edwin Simmons. Various alloys are employed today for strain gages including nickel based, tungsten, palladium, and others. Unfortunately, all of these alloys change resistance with temperature, which causes as “apparent change” in strain. Nickel with its many alloys is the primary strain gage material today. Nickel based wire wound strain gages are used up to about 750 degrees F, but are limited by alloy stability . Foil strain gages are used to 600F but above that experience backing failure and epoxy degradation in test / operation. Due to material characteristics, above 750F the gage factor of strain gage alloys such as MCrAlY or NiCoCrAlY becomes non-linear due to both the temperature and rate of temperature change. A large body of work has been done over the past 40 years to try to develop a simple reliable strain gage alloy that is repeatable for static strain measurements up to 2000 F. Unfortunately irreproducible alloy behavior and high apparent strain due to temperature has precluded any large successes.
Current Sensor Needs
The engine community would like to have a single quarter arm bridge resistance strain gage that can easily installed on engine hot section components and operate in a Wheatstone bridge completion circuit data system.
Ongoing strain gages development work within the OEM community, small business and academia SAB members, and agencies such as NASA GRC are conducting development efforts to identify and qualify new strain gage materials which will allow static strain measurements to be conducted at temperatures over 750F. Reliable repeatable strain gage behavior with minimal temperature induced apparent strain is the goal.
Optics and optical sensing methods provide a new approach to the acquisition of static strain measurements using approaches such as Fiber Bragg Grating (FBG) and Fabry Perot Interferometer (FPI) technologies. This technique holds promise but requires higher temperature fiber materials, packaging, the ability to install the optical sensor into the engine environment, routing fibers out of the engine and data signal processing /acquisition. Ultimately sensor durability reliability and repeatability will be the differentiator.
Strategic Advisory Board (SAB) Members Addressing This Need
Aerodyn Engineering, Inc.
Cleveland Electric Laboratories Company, Inc.